Tbx18-dependent differentiation of brown adipose tissue-derived stem cells toward cardiac pacemaker cells

G9a inhibition promotes the formation of pacemaker-like cells by reducing the enrichment of H3K9me2 in the HCN4 promoter region

Biological pacemakers, made of pacemaker-like cells, are promising in the treatment of bradyarrhythmia; however, the inefficiency of stem cell differentiation into pacemaker-like cells has limited their clinical application. Previous studies have reported that histone H3 at lysine 9 (H3K9) methylation is widely involved in the proliferation and differentiation of cardiomyocytes, but the specific role of H3K9 dimethylation (H3K9me2) in the formation of pacemaker cells remains unclear. The present study evaluated the functional role of H3K9me2 in the differentiation of bone marrow mesenchymal stem cells (BMSCs) into pacemaker-like cells. Rat BMSCs pretreated with the euchromatic histone lysine methyltransferase 2 (G9a) inhibitor BIX01294 were transfected with a T-box 18 overexpression plasmid to induce BMSCs to form pacemaker-like cells. The induced pacemaker-like cells were analyzed using reverse transcription-quantitative PCR (RT-qPCR) and immunofluorescence to assess the efficiency of differentiation. The enrichment of H3K9me2 in the hyperpolarized-activated cyclic nucleotide-gated cation channel (HCN)4 promoter region was assessed by chromatin immunoprecipitation (ChIP). In addition, BIX01294 was injected into rats, and the protein and mRNA expression levels of HCN4 were assessed using western blotting and RT-qPCR. After interference with G9a using BIX01294, ChIP results demonstrated that H3K9me2 levels in the promoter region of HCN4 were markedly decreased. Immunofluorescence and RT-qPCR demonstrated that the protein expression levels of certain cardio-specific proteins in the treated group were significantly higher compared with those in the untreated group. In vivo experiments demonstrated that interference with G9a could cause pathological hypertrophy. Furthermore, in vitro and in vivo inhibition of G9a could increase the differentiation and proliferation of pacemaker-like cells by decreasing the levels of H3K9me2 in the promoter region of HCN4 gene.

Reciprocal interaction between IK1 and If in biological pacemakers: A simulation study

Pacemaking dysfunction (PD) may result in heart rhythm disorders, syncope or even death. Current treatment of PD using implanted electronic pacemakers has some limitations, such as finite battery life and the risk of repeated surgery. As such, the biological pacemaker has been proposed as a potential alternative to the electronic pacemaker for PD treatment. Experimentally and computationally, it has been shown that bio-engineered pacemaker cells can be generated from non-rhythmic ventricular myocytes (VMs) by knocking out genes related to the inward rectifier potassium channel current (IK1) or by overexpressing hyperpolarization-activated cyclic nucleotide gated channel genes responsible for the "funny" current (If). However, it is unclear if a bio-engineered pacemaker based on the modification of IK1- and If-related channels simultaneously would enhance the ability and stability of bio-engineered pacemaking action potentials. In this study, the possible mechanism(s) responsible for VMs to generate spontaneous pacemaking activity by regulating IK1 and If density were investigated by a computational approach. Our results showed that there was a reciprocal interaction between IK1 and If in ventricular pacemaker model. The effect of IK1 depression on generating ventricular pacemaker was mono-phasic while that of If augmentation was bi-phasic. A moderate increase of If promoted pacemaking activity but excessive increase of If resulted in a slowdown in the pacemaking rate and even an unstable pacemaking state. The dedicated interplay between IK1 and If in generating stable pacemaking and dysrhythmias was evaluated. Finally, a theoretical analysis in the IK1/If parameter space for generating pacemaking action potentials in different states was provided. In conclusion, to the best of our knowledge, this study provides a wide theoretical insight into understandings for generating stable and robust pacemaker cells from non-pacemaking VMs by the interplay of IK1 and If, which may be helpful in designing engineered biological pacemakers for application purposes.

Biological pacemaker: from biological experiments to computational simulation

Pacemaking dysfunction has become a significant disease that may contribute to heart rhythm disorders, syncope, and even death. Up to now, the best way to treat it is to implant electronic pacemakers. However, these have many disadvantages such as limited battery life, infection, and fixed pacing rate. There is an urgent need for a biological pacemaker (bio-pacemaker). This is expected to replace electronic devices because of its low risk of complications and the ability to respond to emotion. Here we survey the contemporary development of the bio-pacemaker by both experimental and computational approaches. The former mainly includes gene therapy and cell therapy, whilst the latter involves the use of multi-scale computer models of the heart, ranging from the single cell to the tissue slice. Up to now, a bio-pacemaker has been successfully applied in big mammals, but it still has a long way from clinical uses for the treatment of human heart diseases. It is hoped that the use of the computational model of a bio-pacemaker may accelerate this process. Finally, we propose potential research directions for generating a bio-pacemaker based on cardiac computational modeling.

Genetically Modified Porcine Mesenchymal Stem Cells by Lentiviral Tbx18 Create a Biological Pacemaker

Background

Tbx18 is a vital transcription factor involved in embryonic sinoatrial node (SAN) formation process but is gradually vanished after birth. Myocardial injection of lentiviral Tbx18 converts cardiomyocytes into pacemaker-like cells morphologically and functionally. In this in vitro and in vivo study, genetical modification of porcine bone mesenchymal stem cells (BMSCs) by recapturing the Tbx18 expression creates a biological pacemaker which was examined.

Methods

The isolated porcine BMSCs were transfected with lentiviral Tbx18, and the induced pacemaker-like cells were analyzed using real-time polymerase chain reaction and western blotting to investigate the efficiency of transformation. Then, the induced pacemaker-like cells were implanted into the right ventricle of the SAN dysfunction porcine model after the differentiation process. Biological pacemaker activity and ectopic pacing region were tested by an electrocardiograph (ECG) monitor.

Results

The isolated porcine BMSCs expressed specific surface markers of stem cells; meanwhile, the expression of myocardial markers was upregulated significantly after lentiviral Tbx18 transfection. The porcine SAN dysfunction model was constructed by electrocoagulation using a surgical electrotome. The results showed that the mean heart beat (HR) of BMSCs-Tbx18 was significantly higher than that of BMSCs-GFP. An ectopic pacing region was affirmed into the right ventricle by ECG after implantation of BMSCs-Tbx18.

Conclusion

It was verified that Lenti-Tbx18 is capable of transducing porcine BMSCs into pacemaker-like cells. Genetically modified porcine BMSCs by lentiviral Tbx18 could create a biological pacemaker. However, further researches in large-scale animals are required to rule out unexpected complications prior to application in clinical practice.

Specific Cell (Re-)Programming: Approaches and Perspectives

Many disorders are manifested by dysfunction of key cell types or their disturbed integration in complex organs. Thereby, adult organ systems often bear restricted self-renewal potential and are incapable of achieving functional regeneration. This underlies the need for novel strategies in the field of cell (re-)programming-based regenerative medicine as well as for drug development in vitro. The regenerative field has been hampered by restricted availability of adult stem cells and the potentially hazardous features of pluripotent embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs). Moreover, ethical concerns and legal restrictions regarding the generation and use of ESCs still exist. The establishment of direct reprogramming protocols for various therapeutically valuable somatic cell types has overcome some of these limitations. Meanwhile, new perspectives for safe and efficient generation of different specified somatic cell types have emerged from numerous approaches relying on exogenous expression of lineage-specific transcription factors, coding and noncoding RNAs, and chemical compounds.It should be of highest priority to develop protocols for the production of mature and physiologically functional cells with properties ideally matching those of their endogenous counterparts. Their availability can bring together basic research, drug screening, safety testing, and ultimately clinical trials. Here, we highlight the remarkable successes in cellular (re-)programming, which have greatly advanced the field of regenerative medicine in recent years. In particular, we review recent progress on the generation of cardiomyocyte subtypes, with a focus on cardiac pacemaker cells. Graphical Abstract.

TBX18 transcription factor overexpression in human-induced pluripotent stem cells increases their differentiation into pacemaker-like cells

BACKGROUND

The discovery of gene- and cell-based strategies has opened a new area to investigate novel approaches for the treatment of many conditions caused by cardiac cell failure. The TBX18 (T-box 18) transcription factor is considered as a prominent factor in the sinoatrial node (SAN) formation during the embryonic development. In this in vitro study, the effect of TBX18 gene expression on human-induced pluripotent-stem-cell-derived cardiomyocytes (hiPS-CMs) to induce pacemaker-like cells was examined.

METHODS

The human-dermal-fibroblast-derived iPSCs were transfected using chemical, physical, and Lentiviral methods of TBX18 gene delivery during differentiation into cardiomyocytes (CMs). After the differentiation process through small-molecule-based temporal modulation of the Wnt signaling pathway, the hiPSC-CMs were analyzed using the real-time polymerase chain reaction, immunocytochemistry, immunofluorescence, whole-cell patch-clamp recording, and western blotting to investigate the accuracy of differentiation and identify the effect exerted by TBX18.

RESULTS

The hiPS-CMs showed spontaneous beating and expressed specific markers of cardiac cells. The lentiviral-mediated TBX18 delivery was the most efficient method for transfection. The results showed the increment in Connexin 43 expression among untransfected hiPS-CMs, whereas this protein was significantly downregulated followed by TBX18 overexpression. TBX18-hiPSCMs were detected with pacemaker cell features.

CONCLUSIONS

It was demonstrated that the TBX18 gene is able to conduct hiPSCs to differentiate into pacemaker-like cells. The TBX18 gene delivery seems to have the potential for the development of biological pacemakers; however, more investigations are still needed to assess its usefulness to fix arrhythmic conditions with SAN failure basis.

Comparison of mouse brown and white adipose‑derived stem cell differentiation into pacemaker‑like cells induced by TBX18 transduction

The present study aimed to compare brown adipose-derived stem cell (BASC) and white adipose-derived stem cell (WASC) differentiation into pacemaker‑like cells following T‑box (TBX)18 transduction. Mouse BASCs and WASCs were induced to differentiate into pacemaker‑like cells by adenovirus‑TBX18 transduction in vitro. The transduction rate was determined by fluorescence microscopy and cell ultrastructural changes were observed by transmission electron microscopy at 48 h post‑transduction. The mRNA and protein expression of pacemaker cell‑associated markers, including TBX18, TBX3, sarcomeric α‑actinin (Sr) and hyperpolarization‑activated cyclic nucleotide‑gated channel 4 (HCN4), were detected by reverse transcription‑quantitative polymerase chain reaction, immunofluorescence staining and western blot analysis. The results demonstrated that no significant difference was observed in the transduction rate between BASCs and WASCs. The ultrastructure of BASCs was observed to be more complex than that of WASCs, indicating that BASCs may possess a better structural foundation to differentiate into pacemaker‑like cells. TBX18, TBX3, Sr and HCN4 mRNA and protein expression in differentiated stem cells was significantly increased compared with the respective control groups. Furthermore, the expression levels were significantly higher in TBX18‑BASCs compared with TBX18‑WASCs. In conclusion, TBX18 gene transduction may facilitate the differentiation of BASCs and WASCs into pacemaker‑like myocardial cells, and BASCs may have a higher capacity than WASCs for this differentiation. TBX18 gene may therefore act as an efficient candidate in cell transplantation therapy for diseases and for future research into the cardiovascular system.